A number of methods are known and have been made commercially available in the well logging art for studying the radioactive properties of earth formation, both where the radioactivity is natural and where it is artificially induced. Logs of such properties aid immeasurably in the study of the nature of the subsurface formation, particularly in exploration for mineral or petroleum deposits of such quantities as to make them commercially inviting. One such method relates to the analysis of the energy spectrum of gamma radiation artificially produced in formations adjacent a borehole.